When Does Autumn Begin?

Most people consider the first day of autumn to be the Autumnal Equinox, which this year is September 22. This is because, astronomically, the sun will be directly overhead of the Equator as the Earth`s tilt begins to point the northern hemisphere away from the sun.

As the earth revolves around the sun, at different times of the year the sun will be situated directly overhead at mid-day within the tropics. The Autumnal Equinox is the day the sun is again highest in the sky at noon over the equator as the apex progresses south. The day the sun is straight up at noon over the Tropic of Cancer and Capricorn (23.5 degrees North and South latitude) are the summer and winter solstices, respectively.

Many refer to Astronomical Autumn on September 22 as the first "official" day of autumn. Most meteorologists will argue that autumn has been well under way by then and they have been enjoying several weeks -- if not longer -- of "fall" already.

Meteorologists observe seasons over different time periods. Meteorological fall in the northern hemisphere`s temperature zones begins on September 1, winter begins December 1, spring begins on March 1 and summer begins June 1.

There are a couple of very important reasons why this is the case. The most important is for climate record-keeping. Climatologists require set time periods to calculate averages and do year-to-year and decade-to-decade seasonal comparisons. Astronomical dates will fall on different days depending on the year and keeping seasonal climate records based on those dates would be confusing and inaccurate.

A second reason is that weather-wise, it makes more sense around the globe. In autumn, the heat of summer is waning in September just as cold, freezing temperatures become more common by December 1.

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